Television video-frequency signaltranslating system



Patented July 1, 1941 TELEVISION VIDEO-FREQUENCY SIGNAL- TRANSLATING SYSTEM Harold M. Lewis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation Dela- WHO Application august 31, 1938, Serial No. 227,629

10 Claims.

amplitude range in order to procure apparent optimum fidelity of reproduction of scenes being transmitted and received. This distortion is de sirable due to certain inherent characteristicsof television apparatus as well as to certain physiological phenomena. More particularly, in conventional television apparatus some of the component elements, such as amplifiers and cathoderay tubes and the like. haVednput'a-output or stimulus-response characteristics which deviate from linearity to such an extent as to result in appreciable distortion in the reproduced signal unless compensated for. Further, if the brightness of each element of the reproduced television image is proportional to the brightness of the corresponding point of the original image, the reproduced image may appear flat and uninteresting to an observer. .This may be due, in part, to the fact that the reproduced image ordinarily appears in black and white only and increased detail effects, such as are given to lighter portions of a picture .by its natural colors, are lost. Various devices have heretofore been proposed for distorting video-frequency signals so to reproduce images that the contrast of either their lighter or darker portions is increased over that of the other portions.. In general, such devices are objectionable in that their operations directly affect the over-all gain of the system; that is, the average brilliance of the reproduced scene.

In photography a film is said to have a "gamma which deviates from unity in accordance with differences in the relative detail in the film for the brighter 'anddarker portions of the scene compared to other portions thereof- The same concept arises in television. The gamma" of any reproduction is defined as the tangent of the stimulus-response curve plotted on a logarithmic scale. Obviously only where gamma is unity is there a linear relationship between the stimulus and response over the entire response range of the system.

It is an object of the present invention to provide an improved method of, and apparatus for, effecting a predetermined distortion of a videofrequency signal in a television system, whereby improved detail effects over a selected portion of the signal-amplitude range may be procured.

It is a further object of the present invention to provide an improved method of, and apparatus for, procuring optimum over-all gamma in a television system.

In accordance with the invention, there is provided a television video-frequency signal-translating channel comprising, signal repeating means having an adjustable repeating characteristic with respect to a part only of the amplitude range 01' a signal translated by the channel, a second signal-repeating means having anadjustable repeating ratio with respect to the total 0! the amplitude range. a single control means for simultaneously eiiecting adjustments of the two signal-translating means in opposite senses, and an output circuit so coupled to the repeating means that the signal output of the channel is anected by both the adjustments.

For a better understanding of the invention. to gether with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

in the drawing, Fig. 1 is a schematic diagram of a complete television signal-receiving system including a video-frequency amplifier embodying the present invention; Fig. 2 is a group of curves illustrating certain operating characteristics of the system of Fig. 1, to aid in the understanding of the invention; Fig. 3 is a circuit diagram of a modified form of the video-frequency amplifier of the present invention; and Fig. 4 is a group of curves illustrating th operating characteristics of the circuit of Fig. 3.

Referring now more particularly to Fig. 1, the system there illustrated comprises a television receiver of the superheterodyne type including an antenna system in, II connected to a radiofrequency amplifier l2 to which there is connected in cascade, in the order named, an oscillator-modulator I3, an intermediate-frequency amplifier I4, a detector and A. V. C. supply It, a video-frequency amplifier indicated generally at l6 and embodying the present invention and a signal-reproducing device H, such as a cathode-ray reproducing tube. A line-frequency scanning-wave generator [8 and a field-frequency scanning-wave generator l9 are also coupled to the output of th detector [5 and to the scanning elements of the signal-reproducing device. The stages l2-l9, inclusive, excepting the videofrequency amplifier I6, may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the general operation of the receiving system just described, television signals intercepted by the antenna circuit Ill, ii are selected and amplified in the radio-frequency amplifier l2 and supplied to the oscillatormodulator l3, wherein they are converted to intermediate-frequency signals which, in turn, are selectively amplified in the intermediate-frequency amplifier l4 and delivered to the detector I. The modulation components oi the signal are derived by the detector I and are supplied to the video-frequency amplifier l8, wherein they are amplified and translated by the apparatus of the present invention, as will be presently further described, and from which they are applied to a brilllancy-control element of the reproducing device II. The intensity of the scanning beam of device I1 is modulated or controlled in accordance with the video-frequency voltages impressed upon the control elements in the usual manner. A control-bias voltage developed by the A. V. C. supply i5 and proportional to the average carrier amplitude, independent of its light modulation, is supplied in the usual manner to control grids of tubes in the stages l2-ll, inclusive, for maintaining the signal-output amplitude of the amplifier I4 within a relatively narrow range for a wide range of received signal intensities. The modulation si nal is also applied to the generators. l8 and I9 and the synchronizing components of the signal are utilized therein to synchronize the operations of these generators with the corresponding scanning apparatus at the transmitter. Saw-tooth current or voltage scanning waves are generated by the generators I8 and I8 and these waves are applied to the scanning elements of the device I! to produce scanning fields, thereby to deflect the scanning beam in two directions normal to each other so as to trace successive series oi parallel lines or fields on the target of the reproducing device to reconstruct the image.

Referring now more particularly to the portion of the system oi. Fig. 1 embodying the present invention, for the purpose of increasing the contrast of the fine detail structure of certain portions of the reproduced image relative to that of other portions thereof, the video-frequency amplifier i6 is designed according to the present invention and comprises a signal-translating channel including repeating means, for example, a pair of vacuum-tube amplifiers and 2|, preferably of the pentode type. The wntrol grid of the tube 20 is connected directly to the output circuit of the detector l5, while an adjustable unbypassed resistor 22 is included in its cathode circuit. Due to the direct connection 'to the detector the signal impressed on .the ampliher 20 is stabilized, that is, the peaks of the synchronizing pulses and, hence, all signal voltage amplitudes corresponding to a particular shade have the same level. The control grid-cathode circuit of the tube 2] is connected across resistor 22, while an adjustable-bias battery 23 is included in its cathode circuit. A common anode circuit is provided for the tubes 20 and 2| comprising a load resistor 24 and inductance element 25 in series. The input circuit of the reproducing device i1 is connected across the load circuit 24, 25 by way of a suitable coupling condenser 26. Operating potentials are supplied to the screen and anodes of the tubes from suitable sources indicated at +80 and +3.

The operation of the amplifier or video-frequency translating channel l6 may best be described with reference to the curves of Fig. 2, in which the curves B represent the plate current grid-voltage characteristics of the tubes 20 and 2i. Curve 0 represents the load or output-volt= age plate current characteristic of tube 2!] alone, while curve D represents the effective corresponding characteristic of tubes 20 and 2i combined. Tube 20 has a substantially linear gridvoltage plate current characteristic as represented by the curve Ipi which corresponds to the condition that the value of resistor 22. is reduced to zero. Therefore, with an applied signal having a wave form such as indicated at V the output voltage of the tube 2! alone is of a substantially undistorted wave form, as indicated at V0. Curve I' i illustrates the corresponding characteristic of tube 2| with the value 01 the resistor 22 and, hence, the negative regeneration of the tube, increased to some predetermined value. Curve In represents the eilective gridvoltage plate current characteristic of the tube 2] and is similar to curve 191 of tube 20 except that its cathode-bias battery 22 is normally adjusted to set the cutoi! point at a predetermined value of signal voltage greater than that oi the tube 20 so that only a predetermined desired upper portion oi the amplitude range or an applied signal is repeated by the tube 2i. The term "eflective grid-voltage plate current characteristic" is used to denote the characteristic with respect to a given reference signal-input voltage V taking into consideration the fact that the actual amplitude 01 the signal input to tube 2| will be diflerent from that to tube 20.

Assuming that the value of resistor 22 is adjusted to impart to the tube 20 the characteristic I' i and to impress on tube 2|, which has a bias of such value as to give it an eilfective characterlstic Ip2, a signal proportional to the value of the resistor 22, the eflective composite inputvoltage plate current characteristic for the two tubes operating in parallel is illustrated by the curve I i-i-I i, while the curve V'o illustrates the resultant output voltage of the channel with the tubes adjusted to have the characteristics illustrated.

An increase of the value of the resistor 22 results in an increased input voltage to the tube 2|, but at the same time the resultant increase of the negative regeneration of the tube 20 reduces its gain so that the over-all amplitude range of the signal output of the channel, therefore, remains substantially constant for all normal adjustments of resistor 22. That is, for a given range of signal amplitude VI, the range of the resultant plate current I i-i-I i is the same as the total amplitude range of the plate current Ipl oi tube 20 when operating alone, as shown by the equal amplitudes of the output voltage swings in curves V'o and V0.

With the applied signal poled as indicated at V; in Fig. 2, it will be seen that the resultant or effective repeating ratio of the repeating means included in amplifier IS, with respect to the part of the amplitude range of the translated signal corresponding to black is increased relative to the repeating ratio with respect to the total amplitude range, as indicated in curve V'o. That is, the control means simultaneously efiects adjustment of the repeating ratios of the two tubes 20 and 2i in opposite senses. The portions of the signal voltage and the contrast and detail corresponding to black, therefore, are substantially expanded, while the other portions of the signal voltage are contracted, so that the over-all gain and, hence, over-all signal-output, amplitude range remain constant. Obviously, if the signal as applied to the amplifier i6 is oppositely poled, the portion of the signal voltage corresponding to white will be expanded. While the character-' istics of both of the tubes 20 and 2| may have the same slope, the characteristic of the tube 21 preferably will have a greater slope than that of the tube 20, to provide a distinct bend in the composite characteristic with a sharp cutofl therefor. It is to be noted that the curves of both Figs. 2 and 4 are simply explanatory and not intended to show the exact operation. It will stage may be connected in cascade with the stage It so that the other end of the amplitude range may also be similarly expanded. While the arrangement of Fig. 1 shows one manner of connecting the two tubes for the purpose explained, it will be appreciated that any suitable manner of individually applying the signal voltages'to the input electrodes of the tubes and varying their repeating ratios in opposite senses may be employed.

In summary, therefore, it is seen that the unit I 6 of Fig. 1 comprises a signal-repeating means including vacuum-tube amplifier 2| having an adjustable repeating characteristic with respect to a part only of the amplitude range oi! the signal translated by the channel, as represented by curve Ip2 of Fig. 2. Specifically, the means for adjusting the repeating, characteristic of tube 2| comprises the adjustable tap on resistor 22 for adjusting the signal-input amplitude to the tube 2i and the amplifier 2| is so biased by battery 23 as to have an adjustable repeating characteristic with respect to only a part oi the amplitude range of the signal translated by the channel due to the fact that the tube is operated beyond cutof! for certain signal amplitudes. as shown by the relation of curve V: to the operating characteristic I of Fig. 2. The vacuum-tube amplifier 20 comprises a second signal-repeating means effectively having an adjustable repeating ratio, by virtue of adjustable cathode resistor 22, with respect to the total of the amplitude range of the translated signal, as represented by curves Ipl and P 1 of Fig. 2. The adjustable tap of resistor 22, therefore, comprises a single control means for simultaneously effecting the two above-mentioned adjustments in opposite senses. Resistor 24 is included in the output circuits of both of tubes 2|] and 2| so that the output circuits are effectively in parallel and the signal be appreciated that, if desired, an additional given value of anode voltage-is illustrated by 191 and, with the bias voltage of the tube 2ia properly adjusted, its characteristic is as shown by the curve I z, the composite or resultant effective characteristic of the amplifier being as indicated by the full-line curve Ipi+Ipz; that is, the tube 2! has a. relatively highrepeating ratio for one predetermined portion of the total signal-amplitude range and the tube a has a relatively high repeating ratio for a different predetermined portion of the total of the amplitude range, the relatively high repeating ratios for the two tubes output of the channel is aflected by both of the adjustments.

Referring'now to the modification of the invention illustrated in Fig. 3, there is provided an amplifier or signal-translating channel Ilia which may be substituted for the amplifier i6 oi! Fig. 1. The modified arrangement includes tubes 20a and 2ia having a common output circult 24a, 25a and a biasing battery 23a for tube 2Ia, all of which are similar to the corresponding' elements of the amplifier i6. Here, however, no resistor is included in the cathode circuit of the tube 20a; the control grid of the tube 2la being connected directly with the control grid of the tube 20a to the output circuit of the preceding stage of the system. For the purpose of imparting to tube 20a a mutual conductance characteristic such that this tube is saturated for input voltage amplitudes above a predetermined value, an adjustable voltage source, such as a battery 21, is included in its anode circuit for reducing its anode voltage. For the purpose of simultaneously controlling the voltages supplied by the batteries 21 and 23a, a suitable unicontrolled device indicated at U is provided connected, for example, to the adjustable tap on the batteries.

The operating characteristics of the circuit of being at opposite ends of the total amplitude range. Curve V0 represents the composite out,- put voltage of the amplifier |6a when the applied voltage is of a wave form, such as is shown at V for one adjustment of the batteries 23 and 21. It will be apparent that the curve Ip1+Ip2, shown at B in Fig. 4, has an intermediate portion, corresponding to the middle of the amplitude range of the translated voltage, which has a very gradual slope while the end portions have a very steep slope corresponding to a high repeating ratio. It will be seen from curve Vs that both end portions of the amplitude range of the translated signal, in other words, both the black and the white portions, are expanded, while the intermediate portion is contracted.

By adjustment of. the unicontrol device suitably to adjust the batteries 21 and 23a, the curves Ipl and Ipz may be shifted, as indicated in Fig. 4 by the dot-dash lines, so that the portion of gradual slope in the resultant or composite curve is correspondingly adjusted, as indicated in Fig. 4, while the output voltage over-all amplitude range remains unchanged. The curve V'o represents the wave form of the over-all output voltage when such adjustments have been made.

It will be apparent that various adjustments of relative detail, or gamma, may be effected in accordance with the present invention to distort predetermined portions of the signal amplitude range without affecting the over-all amplitude range, thereby to provide apparent optimum fidelity of reproduction of the image transmitted and to compensate for the various factors tending to result in distorted reproduction which have been mentioned above.

While there has been .described what is at fications may be made therein without departingfrom the invention, and it is; therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A television video-frequency signal-translating channel comprising, a pair of vacuumtube amplifiers, an input circuit for one of said amplifiers including an unbiased. cathode resistor, an input circuit for the other of said tubes connected across said resistor, said tubes having mutual conductance characteristics and being so biased that said one tube repeats the signal input thereto over its entire amplitude range and said other tube repeats the signal input thereto over only a part of its amplitude range, means for adjusting said resistor simultaneously to adjust the negative regeneration of the first of said tubes and the signal input of the second of said tubes, and a common anode circuit for said amplifiers.

2. A television video-frequency signal-translating channel comprising, signal-repeating means having an adjustable repeating characteristic with respect to a part only of the amplitude range of a signal translated by said channel. a second signal-repeating means having an adjustable repeating ratio with respect to the total 01' said amplitude range, a single control means for simultaneously eil'ecting adjustments of said two signal-translating means in opposite senses, and an output circuit so coupled to said repeating means that the signal output 01 said channel is aflectcd by both said adjustments.

3. A television video-frequency signal-translating channel comprising, a signal-repeating means including means for adjusting the signal-input amplitude thereto for adjusting a repeating characteristic with respect to a part only of the amplitude range of the signal translated by said channel, a second signal-repeating means having an adjustable repeating ratio with respect to the total of said amplitude range, a single control means for simultaneously effecting adjustments of said two signal-repeating means in opposite senses, and an output circuit so coupled to said repeating means that the signal output of said channel is afiected by both said adjustments.

4. A television video-frequency signal-translating channel comprising, a vacuum-tube amplifier having an adjustable repeating characteristic with respct to a part only of the amplitude range oi the signal translated by said channel, a second 'vacuum-tube amplifier having an adjustable repeating ratio with respect to the total of said amplitude range, a single control means for simultaneously effecting adjustments of said two amplifiers in opposite senses, and an output circuit so coupled to said amplifiers that the signal output of said channel is afl'ected by both said adjustments.

5. A television video-frequency signal-translating channel comprising, a vacuum-tube amplifier having such a mutual conductance characteristic and so biased as to have an adjustable repeating characteristic with respect to a part only of the amplitude range of a signal translated by said channel, a second vacuumtube amplifier having such a mutual conductance characteristic and so biased as to have an adjustable repeating ratio with respect to the total of said amplitude range, a single control means for simultaneously eifecting adjustments of said two vacuum-tube amplifiers in opposite senses. and an output circuit so coupled to said amplifiers that the signal output of said channel is affected by both said adjustments.

6. A television video-frequency signal-translating channel comprising, signal-repeating means having an adjustable repeating characteristic with respect to apart only of the amplitude range of a signal translated by said channel, a second signal-repeating means having an adjustable repeating ratio with respect to the total of said amplitude range, a single control means for simultaneously efiecting adjustments of said two signal-translating means in opposite senses, and an output circuit coupled to said amplifiers, said adjustments being so proportioned that the maximum signal output of said channel remains substantially constant for all said adjustments.

7. A television video-frequency signal-translating channel comprising, a signal-repeating means having an adjustable repeating characteristic with respect to a part only of the amplitude range of the signal translated by said channel and having a relatively high repeating ratio for one predetermined portion 01 the total signal-amplitude range with respect to the remainder of said range, a second repeating means having a relatively high repeating ratio for a diilerent predetermined portion of the total of. said amplitude range, a single control means for simultaneously effecting adjustments of said two signal-translating means in opposite senses, and an output circuit so coupled with said repeating means that the signal output of said channel is affected by both said adjustments.

8. A television video-frequency signal-translating channel comprising, a signal-repeating means having an adjustable repeating characteristic with respect to a part only of the amplitude range 01 the signal translated by said channel and having a relatively high repeating ratio for one predetermined portion oi! the total signal-amplitude range with respect to the remainder of said range, a second repeating means having an adjustable repeating ratio for a different predetermined portion of the total of said amplitude range, said signal-repeating means having relatively high repeating ratios at opposite ends of the total amplitude range, a single control means for simultaneously efiecting adjustments of said two signal-repeating means in opposite senses, and an output circuit so coupled with said repeating means that the signal output of said channel is affected by both said adjustments.

9. A television video-frequency signal-translating channel comprising, a vacuum-tube amplifier having an adjustable repeating characteristic with respect to a part only of the amplitude range of the signal translated by said channel, a second vacuum-tube amplifier having an adjustablerepeating ratio with respect to the total of said amplitude range, a single control means for simultaneously effecting adjust ments of said two signal-translating means in opposite senses, and an output circuit so coupled to said repeating means that the output circuits of said amplifiers are in parallel and the signal output of said channel is affected by both said adjustments.

10. A television video-frequency signal-translating channel comprising, a vacuum-tube amplifier having an adjustable repeating ratio with respect to a part only of the amplitude range of the signal translated by said channel and so biased that said amplifier is cut off for input voltages below a predetermined amplitude value, a second vacuum-tube amplifier having an adjustable repeating ratio with respect to the total amplitude range and so biased that said second amplifier is saturated for input voltages above a predetermined value, a single control means for simultaneously effecting adjustments of said two amplifiers in opposite senses, and an output circuit so coupled to said amplifiers that the signal output of said channel is affected by both said adjustments.

HAROLD M. LEWIS. 

